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100 »AY AND NIGHT.
They are said to be in opposition, when their longitudes differ half a circle, or they are in opposite sides of the heavens.
Questions.—1. What is the ecliptic ?—explain. 2. What is the zodiac ?—explain. 3. What is meant by the plane of the earth's orbit ? 4. What are nodes ? 5. What are the divisions of the ecliptic ? 0. What are the celestial poles ? 7. What is the celestial equator ? 8. How is the axis of the earth situated with regard to the plane of the ecliptic? 9. What are the poles of the ecliptic ? 10. What is the latitude of a heavenly body ? 11. The longitude ? 12. How is the longitude of a heavenly body reckoned and stated ? 13. What example is given ? 14. What is the declination of a heavenly body. 16. Right ascension ? 16. When are two planets said to be in conjunction ? 17. In opposition? [note. The points at which the ecliptic cuts the celestial equator are called the equinoctial points. Those two points of the ecliptic farthest from the equator are called sol'sticcs. Jtp'ogee, that point of the orbit of the moon which is farthest from the earth. Per'igee, that point which is nearest to the earth.] 18. Look at fig. 40. and point out the ecliptic, zodiac, and signs of the zodiac.
Day and Night.
Ver'nal, belonging to the spring.
Intersect', to cut, to divide each other mutually.
By the diurnal motion of the earth, the same phenomena appear as if all the celestial bodies turned round it; so that in its rotation from west to east, when the sun or a star just appears on the eastern side of the horizon, it is said to be rising, and as the earth continues its revolution, it seems gradually to ascend till it has reached its meridian ; here the object has its greatest elevation, and begins to decline till it set, or become invisible on the western side. In the same manner the sun appears to rise and run his course to the western horizon, where he disappears and night ensues, till he again illuminate the same part of the earth in another diurnal revolution. One half of the earth's surface is constantly illuminated, and by the regular motion of the earth on its axis, every place is successively brought into light and immersed in darkness. If the axis of the earth were always perpendicular to the plane of the ecliptic, the days would where be of the same length, and just as long as the
DAY AOT» NIGHT. 101
nights. For an inhabitant at the equator, and one on the same meridian towards the poles, would come into the light at the same time, and, on the other side, would immerge into darkness at the same time. And since the motion of the earth Ib uniform, they would remain in the dark hemisphere just as long as in the light; that is, their day and night would be equal;—the plane of the ecliptic would coincide with the plane of the equator. But as the ecliptic and equator make an angle with each other of twenty-three degrees and twentyeight minutes, or in other words, as the axis of the earth has such an inclination to the plane of its orbit, it is manifest that, except the earth be in that part of its orbit where the ecliptic cuts the equator, an inhabitant at the equator and one on the same meridian towards the poles, will not come into the light at the same time, nor, on the other side, immerge into darkness at .the same time. And since the axis of the earth always preserves the same inclination, they will,—except at the points where the two great circles intersect each other,—remain in the dark and light hemispheres different times ; that is, their day and night will be unequal. The points where the equator cuts the ecliptic are at the beginning of the signs Libra and Aries. The earth is at these points of its orbit, or, as it is commonly said, the sun enters the eign Aries on the twentieth of March, and the sign Libra on the twenty-third of September. Hence at these periods, -and at no others, the days and nights are equal all over the world; and on this account they are called equinoxes ; the first the vernal, -and the second the autumnal equinox. At these seasons, the sun rises exactly in the east at six o'clock, and sets exactly in the west at six o'clock; —the light of the sun is then terminated by the north and south poles, and as all parts of the earth turn round once in twenty-four hours, every place must receive the rays twelve hours, and be deprived of them for the same time. But at other seasons, when the rays of light are not terminated by the north and south poles, but extend over the one and do not reach the other, it must be manifest, from a moment's inspection of the circles drawn on globes, or common maps of the world, that day and night will be unequal in all places except those situated on the equator, where they will be always equal. At the poles there is but one day and one night in a year, each of six months. The sun can ne^""
102 . CHANGES OF THE SEASONS.
shine beyond a pole farther than twenty-three degrees and twenty-eight minutes; for that is the extent of his declination ; and when he has declination from the celestial equator either north or south, he must shine beyond one pole and not to the other; the days, therefore, will be longest in one hemisphere when they are shortest in the other.
The subject of this lesson may be illustrated, by hanging any round body above or below the level of a candle so as to correspond with the sun's declination. It will be seen, that the light shines over one pole and does not reach the other. If the ball be then turned round, it will be observed, that the circles performed by any parts of the surface are unequally divided by the light; that it will be constant day or night near the north pole, as the ball is depressed or elevated, and that all the phenomena will be reversed in the other, or lower hemisphere.
Questions.—1. What phenomena appear from the diurnal motion of the earth ? 2 Under what circumstances would the days and nights be every where of the same length ?—-Why ? 3. Why is not the day and night always equal to an inhabitant at the equator, and to one on the same meridian towards the poles ? 4. At what points does the equator cut the ecliptic ? 5. When is the earth at those points of its orbit ?—and what happens at these periods ? 6. At other seasons ? 7. What is said of day aiid night at the poles ? 8. How may the subject of this lesson be illustrated ? 9. Look at fig. 40, and illustrate the variations in the lengths of the days and nights.
Changes of the Seasons.
Obliq'uity of the Ecliptic, the angle which the ecliptic makes with the equator.
Look nature through, 'tis revolution all;
All change, no death. Day follows night, and night
The dying day. Stars rise and set, and rise.
Earth takes th' example; see, the summer gay,
With her green chaplet and ambrosial flowers,
Droops into pallid Autumn. Winter gay,
Horrid with frost, and turbulent with storm,
Blows Autumn and his golden fruits away ;
Then melts into the Spring. Soft Spring, with breath
Favonian, from warm chambers of the south,
CHANGES OF THE SEASONS. 103
Recals the first. All, to reflourish, fades ;
As in a wheel, all sinks, to reascend :
Emblem of man, who passes, not expires.—Thomson.
The orbit in which the earth revolves in his annual course round the sun is not a circle but an ellipse or oval; and we are more than three millions of miles nearer to the sun in December about the time of the winter solstice, than we are in June about the time of the summer solstice. Now as heat and light from the sun are greater as the distance is less, it is manifest that this circumstance would occasion a variation in the temperature of the air, like that of our seasons, if the equator always coincided with the ecliptic. But the seasons with us, in north latitude, are not in the least degree occasioned by this circumstance, but by the direction in which the sun's rays fall upon us. When they fall perpendicularly, or most nearly so, the season is warmest; and when they -fall most obliquely, or in a slanting manner, the season is coldest. The cause of the difference in the obliquity of the sun's rays is the obliquity of the ecliptic. The effect of obliquity, in regard to rays will be evident, if a board be held perpendicularly before a fire. It will then receive a body of rays equal to its breadth. But if it be placed obliquely, at an angle of forty-five degrees, then only half the rays will fall on its surface, and the other half will pass over it; so it is with the surface of the earth in summer and winter. The circumstance also, that the days are longest, whether in north or south latitude, when the sun's rays fall in the greatest quantity and most directly at any place, contributes much to the warmth of summer and the cold of winter. In northern countries, where the days are eighteen or twenty hours long, or where the sun is above the horizon for any number of days together, the heat of summer is equal to that of any part of the world.
Since the degree of heat from the sun increases as the earth's distance diminishes, and this distance is least when it is summer in south latitude, and greatest when it is summer in north latitude, a greater degree of heat, therefore, must be received in summer in south latitude, than in summer in north latitude. But to compensate for a less degree of heat, the inhabitants in north latitude have longer summers than those in south latitude. For as the sun is not in
104 THE MOON.
the centre of an ellipse but in the focus, the earth must move farther in its orbit in one part of its revolution than in the other. It moves slower also as it is farther from Uie sun; and our summers are found to be eight days longer than the summers in south latitude; that is, between the vernal and autumnal equinoxes there are eight days more, than between the autumnal and vernal.
It is well known .that the degree of heat is not greatest", when the days are longest. We have the warmest weather in the latter part of July, and in the first of August; and our coldest month is January. To account for this it has been stated, that a body <5nce heated does not grow cold again instantaneously, but gradually; now as long as more heat comes from the sun in the day, than is lost in the night, the heat of the earth and air will be daily increasing, and this must evidently be the case for some weeks after the longest day, both on account of the number of rays which fall on a given space, and also from the perpendicular direction of those rays. It is for the same reason, that the warmest part of the day is not, when the sun is at the meridian, but about two or three o'clock in the afternoon.
Questions.—1. When are those who live in north latitude nearest the sun ? 2. What would be the consequence if the equator coincided with the ecliptic ? 3. What occasions the seasons with us ? 4. How may the effect of obliquity in regard to the sun's rays be made evident ? 5. What contributes much to the warmth of summer ? 6. What is said of north and south latitudes as respects the degree of heat ?—Explain. 7. Why is not the degree of heat greatest when the days are longest ? 8. Look -it fig. 40, and illustrate the diversity of the seasons.
Quadrature, the first and last quarter of the moon.
The moon is a secondary planet, revolving round the earth in about twenty-nine days and a half, and is carried with the earth round the sun once a year. Its distance from the earth is about two hundred and forty thousand miles; and it turns pn its axis in the same time that it per